Washington — The US Supreme Court ruled in 2005 that it is cruel and unusual punishment to sentence a juvenile offender to death. On Monday, the high court takes up two cases that seek to extend that same constitutional reasoning to the practice of sentencing juveniles to life in prison without any possibility of parole.

The essence of their argument is that adolescents who commit crimes are still developing as human beings and their characters are still largely unformed. It is wrong to write them off as hopeless, to lock them up and throw away the key, these advocates say.

Supporters of tough criminal sanctions offer a counterargument that teens who commit adultlike crimes should face adult-style punishment. Juvenile transgressors are given opportunities to reform themselves. When those efforts fail and adolescents engage in an escalating pattern of violent crime, sometimes long-term incarceration is the safest alternative for society, proponents say.

Both the Graham and Sullivan cases involve young defendants in Florida who intentionally confronted their victims in violent criminal acts.

In 2003, 16-year-old Terrance Graham and another teen attempted to rob a Jacksonville barbecue restaurant. During the incident, the restaurant manager was struck on the head with a steel bar. Terrance pleaded guilty to battery and attempted armed robbery.

In recognition of his plea, the judge sentenced him to 12 months in county jail and three years' probation, rather than a prison term. But six months after his release from jail, Terrance was arrested for participating in a series of armed home-invasion robberies.

The judge revoked his probation and sentenced him to life in prison. In announcing the life sentence, Judge Lance Day said: "It is apparent to the court that you have decided that this is the way you are going to live your life and that the only thing I can do now is to try to protect the community from your actions."

The other Florida case involves Joe Sullivan, who in 1989 at age 13, allegedly beat, raped, and robbed a woman in her 70s.

Earlier that day while the victim was away, Joe and two other teens broke into her home, stealing jewelry and coins. Later, when the woman came home, Joe and another teen allegedly returned. That's when the beating, rape, and second robbery took place.

He was tried as an adult and convicted. Based on his prior criminal history, he scored 846 points under Florida's sentencing guidelines. The guidelines suggest that a life prison sentence is appropriate for anyone scoring at least 583 points.

Joe's lawyers dispute the rape charge. DNA evidence collected at the crime scene was destroyed by the state after the trial 20 years ago, before DNA testing became routine.

Lawyers for Messrs. Graham and Sullivan appealed their convictions and lost at every level. Then in 2005, the US Supreme Court handed down a 5-to-4 decision in a case called Roper v. Simmons. For the first time, the high court declared the death penalty unconstitutional for juveniles.

No matter how heinous the crime, Justice Anthony Kennedy wrote for the majority, juveniles are less culpable for their criminal activities than adults because they are less mature, less able to control impulses and resist peer pressure, and less able to appreciate the full magnitude of their wrongdoing. In addition, he said, experts are unable to predict which juvenile offenders may successfully reform their lives once they are older. Under such circumstances, death is a disproportionate punishment for juveniles, he said.

"When a juvenile offender commits a heinous crime, the state can exact forfeiture of some of the most basic liberties, but the state cannot extinguish his life and his potential to attain a mature understanding of his own humanity," Justice Kennedy wrote.

Life without parole for a juvenile is similar to the imposition of a death sentence, says Sullivan's lawyer, Bryan Stevenson of the Equal Justice Initiative in Montgomery, Ala. In both cases, the punishment is disproportionate to the moral culpability of the offender, he says.

"A child's character is still in flux," Mr. Stevenson writes in his brief. "They are unfinished products, human works-in-progress.... Their potential for growth and change is enormous. Almost all of them will outgrow criminal behavior."

"Juveniles are more malleable and capable of reform than adults," writes Graham's lawyer, Bryan Gowdy, in his brief. "It is cruel to simply give up on them."

Only two individuals in the United States are serving life prison terms for nonhomicide crimes committed at age 13. Both are in Florida. In addition to Florida, five states have sentenced juveniles to life in prison for nonhomicide crimes: Louisiana, Iowa, California, Nebraska, and South Carolina.

Nationwide, an estimated 2,500 inmates are serving life without parole for crimes committed as juveniles; 109 of those sentences were handed down for nonhomicide crimes.

No national consensus has emerged against the imposition of life-without-parole sentences for juveniles, says Florida Solicitor General Scott Makar. Forty-two states permit such sentences, and 38 of them allow it for nonhomicides, he says.

Mr. Makar acknowledges social science and medical research show that juveniles are developmentally different from adults and thus less culpable for their crimes. But, he says, state lawmakers were aware of these findings when they adopted sentencing laws that include age as a factor.

"Florida's legislature, as well as that of every other state, has closely and repeatedly reviewed juvenile justice and sentencing matters over the past 20 years, when crime rates soared and societal concerns justifiably mounted," he writes. "States must have ongoing flexibility to decide what mix of incapacitation, deterrence, and rehabilitation their criminal justice systems will pursue."

If the high court declares life without parole unconstitutional for juveniles guilty of nonhomicide crimes, Mr. Gowdy says, state lawmakers will retain the ability to mete out long prison terms for juvenile offenders. But they would have to afford an opportunity for parole. In addition, juveniles guilty of homicide would still be eligible for a life sentence without parole.

Texas recently eliminated juvenile life sentences without parole from its sentencing statute. It now offers parole eligibility after 40 years in prison. The Texas law applies to all juveniles facing life in prison, including those guilty of homicide.